1. Field of the Invention
The present invention relates to an inkjet recording apparatus, and in particular, to technology for controlling a recovery operation of a recording head ejecting pigment ink.
2. Description of the Related Art
Recently, inkjet recording apparatuses (inkjet printers) have become common as recording apparatuses for printing and recording images captured by digital still cameras, and the like. An inkjet recording apparatus comprises a plurality of recording elements in a head, the recording head being moved to scan a recording medium while droplets of ink are discharged onto the recording medium from the recording elements, and each time one line of an image is recorded onto recording paper, the recording medium is conveyed through a distance corresponding to one line, this process being repeated, whereby an image is formed onto the recording paper.
Inkjet printers include those which use a short-length serial head, and carry out recording by causing the head to scan in the lateral direction of a recording medium, and those which use a line head in which recording elements are aligned up to a dimension corresponding to the full width of one edge of the recording medium. In a printer using a line head, it is possible to carry out image recording across the full surface of the recording medium, by scanning the recording medium in an orthogonal direction to the direction in which the recording elements are arranged. In a printer using a line head, it is not necessary to provide a conveyance system, such as a carriage, or the like, for causing a short-dimension head to scan, and furthermore, movement of the carriage and complex scanning control of the recording medium also becomes unnecessary. Furthermore, since only the recording medium is moved, it is possible to achieve higher recording speeds in comparison to printers using serial heads.
Inks used in inkjet printers have a characteristic in that when they come into contact with the atmosphere, the ink solvent (principally water in the case of water-based inks, for example), evaporates, and the viscosity of the ink increases gradually as time passes, until eventually, it solidifies. Therefore, at nozzles which have not discharged ink for a long period of time, firstly, the ink in the peripheral region of the nozzle opening (the surface where the ink comes into contact with the air) becomes highly viscose, and gradually, the ink inside the nozzle also increases in viscosity.
If increased viscosity of the ink occurs as described above, then the ink becomes less liable to be discharged from a nozzle which has not discharged ink for a long period of time, and furthermore, a problem arises in that as time passes, it will become impossible to discharge the ink, even if the prescribed pressure is applied.
In order to resolve problems of this kind, preliminary discharge (purge, liquid discharge, air discharge, dummy discharge) is carried out at prescribed time intervals, and a recovery operation is implemented whereby the ink of increased viscosity inside the nozzles is discharged externally, or hardened ink is sucked in by means of a suctioning device, such as a pump.
In the inkjet recording apparatus, recovery processing method and storage medium storing a recovering processing program disclosed in Japanese Patent Application Publication No. 11-192729, the nozzles used in printing the same content are established, and therefore, it is not necessary to perform recovery operation during printing, and control is performed in such a manner that a recovery operation is not carried out during recording, if a plurality of prints of the same content are being made. If, on the other hand, the print contents are different, then the time that each nozzle is left unused is recorded, and if it is equal to or below a reference time, then control is implemented in such a manner that a recovery operation is not carried out for that nozzle. By performing control in this way, the number of preliminary discharge operations is reduced and the amount of ink consumed is also reduced.
On the other hand, inks to be used in inkjet printers are roughly classified into dye-based inks and pigment-based inks. As the coloring material for ink-jet ink, dyes have been used, but they have drawbacks as to water resistance and light resistance. To overcome the drawbacks, pigments have come to be used. Images obtained from pigment inks have remarkable advantages of superior light resistance and water resistance compared with images obtained from dye-based inks. However, there are problems that the pigment is not easily formed uniformly or pulverized into fine-particles of a nanometer size (i.e., monodispersion), which can permeate pores in the surface of paper, and it is then difficult to form images with high saturation because the pigment is poor in permeability into the paper.
As measures to solve the above-described problems, Japanese Patent Application Publication No. 2002-146222 discloses a method to produce fine particles of pigment using a microjet reactor method. This is a method of introducing a solution, in which a pigment is dissolved, and a precipitation medium solution, into two nozzles that are opposite to each other and that have different micrometer sizes, at high pressure (for example, 5 MPa) by means of pumps; and introducing a gas (such as compressed air) perpendicularly into a portion where the jet flows of the two solutions collide with each other, so as to discharge a suspension of the pigment by the gas flow (flow rate, about 0.5 m3/h).
If a recovery operation is carried out frequently in order to prevent discharge errors or discharge failures, then not only does this increase the amount of ink consumed, but furthermore, it also causes the productivity of the actual printing process to fall. Moreover, since a recovery device for performing a recovery operation includes consumables parts, such as a blade, or the like, then it is necessary to carry out maintenance of these consumable parts.
However, it is difficult to solve the problems concerning the discharge errors and the discharge failures with the measures disclosed in Japanese Patent Application Publication No. 11-192729. In particular, when the pigment ink containing the pigment particles of large particle size and poor monodispersion is used, it is difficult to form images with high saturation, and moreover, the discharge errors and the discharge failures may be aggravated.
In the inkjet recording apparatus, the recovery processing method and the storage medium storing a recovery processing program disclosed in Japanese Patent Application Publication No. 11-192729, even in the case of prints of the same content, if a large volume of prints are made at any one time, then any nozzles not used in that print will be unused for a long period of time, and hence the ink inside the nozzles may solidify completely (the viscosity thereof may become extremely high or the viscosity may increase from the nozzle opening section and through to the deeper regions of the nozzle), and it may not be possible to recover the nozzle simply by performing preliminary discharge. Considering the case of a print having long dimensions, if the time out of use is checked before printing, then increase in the viscosity of the ink will occur during printing, and the reference time period until discharge errors arise may be exceeded.
Consequently, discharge errors or discharge failures may occur at the nozzles, and hence, in the print result, streaking may arise in the direction of conveyance of the recorded medium. The aforementioned problem is particularly noticeable in the case of single-pass printing using a line head.
In the method for producing fine particles of pigment disclosed in Japanese Patent Application Publication No. 2002-146222, particles are generated in a very small space, of a micrometer scale, and the particles are immediately taken out from the apparatus, thereby preventing the apparatus from being blocked by the pigment fine-particles. This method is then preferred to obtain fine particles having a narrow particle size distribution. However, the method has such problems as that the contact time for the two solutions is not easily controlled, and thus subtle reaction control is difficult. Therefore, it is difficult to stably produce fine particles of pigment of sufficient monodispersion.
In the related art as described above, there are unsolved problems of recovery of the discharge errors and the discharge failures and production of fine particles of pigment, and it is hence difficult to obtain images of high resolution and high saturation by means of the pigment ink.